A method for enzymatically decomposing a protein raw material by liquid koji, the protein raw material containing a lot of glutamic acid, has been well known to have the glutamic acid content increase in a seasoning liquid (for example, see Patent Documents 1 to 4).
However, the method using those conventional techniques comprises the step of having liquid koji coexist with glutaminase producing microorganisms, or adding an enzyme source such as glutaminase during the enzymatic decomposition. Accordingly, it has been difficult to increase the glutamic acid content in the seasoning liquid without using glutaminase producing microorganisms or an enzyme source such as glutaminase.
Herein, glutaminase producing microorganisms have been known, including Rhodotorula (for example, see Patent Document 5), Bullera (for example, see Patent Document 6) and Cryptococcus (for example, see Patent Documents 7-9) or the like. Further, enzyme sources of glutaminase or the like have been also known, including the enzyme sources derived from Cryptococcus (for example, see Patent Document 7) and yeast (for example, see Patent Document 10).
Meanwhile, in the technological field of producing a soy sauce, a method for increasing a glutamic acid content in a seasoning liquid has been known as described in the following. That is, such a method comprises the steps of: producing solid koji by adding a predetermined amount of zinc ions into koji producing raw materials when solid koji is to be prepared; and producing a soy sauce of which glutamic acid content is greatly high by using the resultant soy sauce koji (for example, see Patent Document 11). Herein, note the above mentioned soy sauce koji has ten-fold higher leucine aminopeptidase activity than the soy sauce made from the koji producing raw materials added with no zinc ions. See, for example, Patent Document 11.
However, when a protein raw material is enzymatically decomposed by liquid koji, no method for adding zinc ions to raw materials has been known yet. According to experiments conducted by the present inventors, when liquid koji was prepared, even though a predetermined amount of zinc ions were included in a liquid culture medium, those experiments demonstrated that the leucine aminopeptidase activity was incapable of being increased. That is, the present inventors found that the effect of increasing the leucine aminopeptidase activity by adding zinc ions as disclosed in Patent Document 11 was restricted to only the case using the solid koji in the technological field of producing a soy sauce.
In the meantime, when a protein raw material is enzymatically decomposed, a method for obtaining protein hydrolysates in a short time and a high yield has been known, comprising the steps of: decomposing a protein raw material by using an acidic protease or a material including an acidic protease under the conditions that the concentration of bivalent metal ions such as calcium ions and magnesium ions becomes at least 0.01 molar or more in the enzymatic decomposition solution within an acidic pH range (for example, see Patent Document 12).
However, Patent Document 12 describes calcium ions and magnesium ions, while the same document describes no bivalent ions besides the above mentioned ions (for example, manganese ions, zinc ions and cobalt ions or the like). Moreover, Patent Document 12 describes an acidic protease or a material including an acidic protease as an exemplary enzyme, while no liquid koji is described therein. Furthermore, Patent Document 12 does not describe or suggest any of the enzymatic decomposition of a protein raw material through combining liquid koji with zinc ions.
As obvious according to the above mentioned explanation, when a protein raw material is enzymatically decomposed by liquid koji, no method for producing a seasoning liquid has been known yet, comprising the steps of: adding a zinc source into an enzymatic decomposition solution under the conditions so that the concentration of zinc ions is set in a predetermined value therein; and increasing a glutamic acid content by the zinc ion addition.